Nonredox Metal-Ions-Enhanced Dioxygen Activation by Oxidovanadium(IV) Complexes toward Hydrogen Atom Abstraction

Dioxygen activation toward efficient catalysis at ambient temperature is still a big challenge for industrial oxidations, while it proceeds smoothly in nature. This work presents an example of that adding nonredox metal ions as Lewis acid can enhance dioxygen activation by oxidovanadium(IV) complex, [V^IV(O)Cl(TPA)]PF₆ (where TPA is tris-[(2-pyridy)methyl]amine), which leads to efficient hydrogen abstraction at ambient temperature, whereas, in the absence of a Lewis acid, the catalytic hydrogen abstraction of the oxidovanadium(IV) complex is very sluggish. Ultraviolet-visible light (UV-vis), electron paramagnetic resonance (EPR), mass, and nuclear magnetic resonance (NMR) studies have provided informative clues to indicate the interaction between the Lewis acid and vanadium complexes, including assisting the dissociation of the chloride from the oxidovanadium(IV) complex, interacting with the vanadium oxido group, and stabilizing the vanadium(V) superoxo species. These interactions enhanced the dioxgyen activation efficiency of oxidovanadium(IV) complex, and improved the hydrogen abstraction ability of vanadium(V) oxido species, which leads to efficient hydrogen abstraction in a catalytic process. A brief mechanism has also been proposed for dioxygen activation toward hydrogen abstraction by an oxidovanadium(IV) complex.